Both serotonergic and non-serotonergic neurons located in the medullary nucleus raphe magnus which project to the spinal cord have been implicated in the regulation of pain sensitivity. Activation of these neurons inhibits the transmission of nociceptive information in the dorsal horn of the spinal cord and results in antinociception. Although the ability of raphe magnus neurons to regulate the transmission of nociceptive information at the spinal level is well-documented, little is known about the inputs which regulate the activity of raphe magnus neurons themselves. Previous work is this laboratory has indicated that neurons in the raphe magnus are subject to modulation by both noradrenergic and cholinergic neurons. The proposed studies are designed to characterize the interactions among serotonergic, noradrenergic, and cholinergic neurons in the brainstem and spinal cord and to determine the functions of these neuronal systems in the regulation of nociception. The proposed studies will involve: (1) microinjecting drugs which modify cholinergic and noradrenergic function in the raphe magnus and determining the resulting effects of nociception; (2) determining the effects of iontophoretically-applied cholinergic and noradrenergic drugs on identified neurons in the raphe magnus; (3) determining the capacity of spinally-projecting serotonergic and noradrenergic neurons to produce antinociception by activating them and modifying the resulting antinociception by spinal injections of drugs which alter monoaminergic function; (4) determining whether the antinociception produced by electrical and pharmacological stimulation of brainstem neurons is mediated by the release of endogenous serotonin and norepinephrine in the spinal cord; (5) determining the anatomical connections of cholinergic, noradrenergic, and serotonergic neurons in the brainstem and spinal cord; and (6) determining the location of noradrenergic receptor subtypes involved in modulating nociception in the brainstem and spinal cord. A detailed study of the pharmacology, neurochemistry, anatomy, and electro- physiology of these neuronal systems should provide important information about the mechanisms by which they regulate the transmission of nociceptive information. These pharmacological studies of pain modulatory systems may provide important basic information which could lead to the development of non-opioid analgesic drugs which do not possess the unfortunate side-effects and abuse potential of currently used opioids.